Calibrated retardation plate for measuring birefringence and method of making same



Feb. 1, 1949. LQWBER r 2,460,515

CALIBRATED RETARDATION PLATE FOR MEASURING BIREFRINGENCE AND METHOD OF MAKING SAME Filed April 25, 1946 2 Sheets-Sheet 1 lab I F Ill |-1|||1 gal fig. 8

IN IEN TOPS DAV/D D. LOWBER 3 2 JOSEPHD FEARDO/V 3y PH/L 0 COL l. YE

A TTOPNE Y SEARCH ROOM Feb. 1, 1949. I D. D. LOWBER ETAL 2,460,515

CALIBRATED RETARDATION PLATE FOR MEASURING BIREFRINGENCE AND METHOD OF MAKING SAME Filed April 23, 1946 2 Sheets-Sheet 2 9o 1 'I/ l l i 1/ n \n 62 179 6 I 0. INVENTOPS DAV/D D. Lowatn JOSEPHD. REARDON PH/L /P can 75/? A I 11$ 7 ATTORNEY Ski-\KUH KUUlVl Patented Feb. 1, 1949 CALIBRATED RETARDATION PLATE FOR MEASURING BIREFRINGENCE AND METH- OD OF MAKING SAME David D. Lowber, Snyder, and Joseph D. Reardon and Philip W. Collyer, Buffalo, N. Y., assignors to American Optical Company, Southbridge, Mass, a voluntary association of Massachusetts Application April 23, 1946, Serial No. 664,392

4 Claims.

This invention relates to optical devices and more particularly to new and improved birefringent retardation plates, wedges and the like employed with optical instruments, such as petrographic and polarizing microscopes, for the study and measurement of certain physical properties of specimens or samples of isotropic and anisotropic materials. The invention also relates to a method of manufacture of such re- 2 member for indicating the magnitudes of the birefringence of specimens under examination as well as the sign of such bifringence. The invention also includes the method of making such a retardation plate.

It is a further object of the invention to provide a plate-like device formed of a plane parallel member and a wedge shaped member cemented or secured together and having biretardation plates, wedges and the like. fringent retardation properties and a r d Heretofore, retardation plates have been ated scale associated therewith for indicating made from various known birefringent matepositive, negative and zero bifringerant values rials, such as quartz, and in order to function of said device, such scale being accurately 10- in the manner desired have been made as relacated relative to the transverse plane of zero tively long, thin tapered plate-like members and birefringence and so graduated as to give acprovided with scales for indicating the amount curate indications of the amounts of positive of birefringence produced by the various thickand negative birefringence in the sample being nesses of such plate-like members. Since the inspected. The invention further includes the amount of birefringence at any particular secmethod of producing such a plate-like device. tion of such a tapered member is directly pro- 2 Other objects and advantages of the invenportional to the thickness at such section, it tion will become apparent from the following was necessary, in order to provide values apdescription taken in conjunction with the acproaching zero, that such a member be tapered companying drawings. It will be understood to an exceedingly thin sharp edge. In fact, that changes may be made in the details of consuch members were t thi t b practlcaL struction, arrangement of parts and the steps Furthermore they would not accurately indiof the method employed without departing from cate zero birefringence, nor both positive and the sp r t o the invention as expressed y the negative values of birefringence. Also it was accompanyin l W t r do o found diflicult to provide such members w th wish to be limited to the exact details of conclear, sharply defined, accurately calibrated struction, arrangement of parts and steps of the durable scales which were correctly located on Process shown and described as the P f r the members to thus afiord the high degree of form has be given y ay of i t tion on y. accuracy desired. Referring to the drawings:

Applicants invention comprises a new m th- Fig. 1 is a side elevational view of a polarizod of making birefringent retardation plateing microscope and retardation plate attachlike members in such a manner as to overcome merit embodying th inven ion; the above objections and in accurately locating 2 1S a P V Of the attachment f and calibrating a scale thereon, and likewise a P t of in r ken wayto mor comprises the new and improved plate-like reclearly show a slide and the retardation plate tardation devices resulting from such method. r in;

It is, accordingly, an object of the present Fig. 3 is a vertical sectional view taken subinvention to provide a plate-like member havstantlally on line 3-3 of Fig. 1 and lookin n progressively varying bifringent retardation in the direction of the w properties along the length thereof and ranging Fig. 4 is a sectional view taken substantially from zero to relatively large values thereof and On lin of Fi 3 and kin i t r having a scale accurately positioned to indicate tion of the arrows; such values. The invention also includes the Fig. 5 is an enlarged plan view of superimmcthod of producing such an accurately caliposed birefringent retardation plate-like membrated member. =bers employed in the slide of Fig. 3;

It is another object of the invention to pro- Fig. 6 is an elevational view of the plate-like vide a plate-like member of the character described having positive, negative and zero bireiringent retardation properties and a graduated scale associated therewith including a base members of Fig. 5;

Fig. 7 is a diagrammatic showing of a portion of the plate-like members 01' Figs. 5 and 6, but greatly distorted in shape in order to more reference mark accurately positioned upon the as clearly describe the invention;

Fig. 8 is a conventional optical system which may be employed in the structure of Fig. 1; and

Fig. 9- is a plan view of the reticule of the instrument and a graduated scale on said plate-like members as viewed through the eyepiece of the instrument of Fig. 1.

Referring to the drawings in detail, numeral indicates generally an optical device or attachment arranged to be secured to the upper end of a conventional polarizing microscope l2 and adapted to function in conjunction with a polarizer and condenser l4 positioned beneath a rotatable stage l6 so that the birefringent properties of specimens or the like, such as minute crystals of material upon a slide l8, may be studied and measured thereby. The attachment in, as is clearly shown by Figs. 2 and 3, comprises a split ring portion 20 which may be clamped by a thumb screw 22 about an upstanding flange (not shown) upon the upper end of the microscope tube 23 and to this ring 20 is secured, as by screws 24, a lower guide plate 26. This plate is in turn secured by screws 21 to a pair of spaced guide rails 28 and an upper plate 29 is secured to the guide rails by screws 30. The plate 29 and guide rails 28 are adapted to form with the plate 26 guideways for the lower lateral corners 3| of a slide 32. The lower plate 26 is provided with an opening 34 and the upper plate 28 is provided with a threaded opening 36 concentrically arranged relative to the optical axis of the microscope l'2 so that a removable collar 38 may be threaded into the opening 36 and serve to support an eyepiece 40 and associated structure in optical alignment with objective of the microscope as will be hereinafter more fully described. An adjustable pin 31 is provided with an eccentric head 39 and a slot 31' so thatit may be rotated for proper engagement with a suitable notch in said upstanding microscope flange and maintains the attachment l0 in a predetermined angular relation relative thereto. A set screw 39' locks pin 31 against rotation.

The slide 32 is of generally rectangular shape. as shown by Fig. 2, and is elongated so that an aperture 42 may be formed therein for accommodating a graduated retardation plate 44 which has its opposite ends fitted into grooves in supporting blocks 46 and 41 (see Fig. 3) arranged in recesses 48 and 49 in said slide adjacent the opposite ends of the aperture 42. These blocks are removably held in place by screws or the like 50. The opposite lateral edges of the slide 32 are bevelled as indicated at 52 to form the lower corner portions 3| previously referred to and these corners guide the slide as it is moved rectilinearly and normally to the optical axis of the microscope. Thus the slide may be moved for selectively positioning various portions of the retardation plate 44 in optical alignment with the optical system of the microscope. Handles 54 are provided at opposite ends of the slide for moving the slide in either direction. Removable covers 56 and are secured by screws 58 at opposite ends of the guide rails 28 and serve to fit closely about the slide 32 and thus enclose the guideway therefor.

Positioned within the removable collar 38 is a centerable cell 60 which serves to support a transparent plate 62 having a reticule 64 comprising cross hairs 64a and 64b (see Fig. 9) formed preferably upon its lower surface. Threaded into this cell 60 is a retaining ring 65 adapted to enclose an analyzer 66 between an upper flange thereof and a flange formed on the cell 60 intermediate said analyzer and the transparent plate 62. An eyepiece supporting and guiding member 68 is provided with a flange 68 threaded into the upper end of the collar 38 and a plurality of centering screws 10 extend through this flange. These screws are arranged to have their sloping inner ends engaged with a bevelled rim portion 12 of the cell 60 so that adjustment of these screws as desired may be utilized to properly center and retain the reticule in place relative to the optical system of the eyepiece 40. An enclosing ring 13 is thereafter threaded into the upper end of the collar 38 and this prevents anyone from disturbing the adjustment of these screws. The tubular guide 68 is provided with a cylindrical inner surface to slidably receive the eyepiece 40 and spring fingers 14 are formed in said tubular guide to provide friction means for maintaining the eyepiece in any desired adjusted position during use of the device. The eyepiece may be of conventional construction so long as it is adapted to focus substantially at the plane of the reticule 64.

The retardation plate or device 44 is formed by a lower plate-like member 18 and an upper plate-like member 18 formed of material having birefringent retardation properties. A preferred material for these members is quartz. As may be clearly seen from Figs. 5 and 6, members 16 and I8 are of relatively thin elongated platelike form. The lower member 16 is accurately formed with optically flat upper and lower surfaces which are disposed in the shape of a relatively thin wedge having its thicker and 88 located at the right and its thinner end 82 located at the left as viewed in Fig. 6. The plate-like member 18, on the other hand, is accurately formed with optically fiat upper and lower parallel surfaces. In order to more clearly emphasize the relative shapes of these two members, an exaggerated condition thereof is digrammatically indicated by Fig. 7 and it will be readily seen I therefrom that while the lower member indicated by numeral 16' is wedge shaped, as viewed from a side thereof, the upper member indicated by 18 is provided with upper and lower parallel surfaces.

The member 16 is cut from quartz in such a manner that the optical axis of the crystal extends in a. direction parallel to the longitudinal dimension of the plate 16, as indicated by the arrow 84, while the member 18 is cut from quartz in such a manner that the optical axis of the crystal extends in the transverse direction of plate 18, as indicated by the number 86. Since the amount of birefringence introduced into a beam of light by a plate-like member normal to said beam is directly proportional to the thickness of the plate-like member, it would be possible to employ merely a single elongated Wedge for introducing various amounts of birefringence, if it were practical to have such a wedge taper down to an extremely thin sharp edge. However, in dealing with quartz and like materials such is highly impractical. Even if such a wedge were feasible only positive or only negative birefringent readings could be obtained thereby, depending upon the particular position of wedge relative to the position of the polarizer. Applicants arrangement, however, provides two plate-like birefringent members with their optical axis at right angles to each other so as to produce a resultant birefringence in a beam of light passing therethrough equal to the diilerence of their combined thickness. The plate-like member 18 SEAKUH KUUM having parallel upper and lower surfaces and the plate-like member 18 being in the form of a relatively thin elongated wedge are arranged in superimposed relation and are of such proportions that said wedge 16 has a transversed section intermediate its ends having a thickness which is the same as the thickness of the platelike member 18. The resultant birefringence at such intermediate transverse plane will equal zero. The difference in thicknesses in the members l8 and 18 at any location to the right side of this plane will thus produce a positive birefringence while the difference in thicknesses at any location therein at the opposite side of said transverse plane will produce a negative birefringence.

It is desirous to provide for such a combined retardation plate 44 an accurately calibrated scale for indicating the various amounts of positive and negative birefringence produced by the retardation plate. It is also desirable to accurately locate such a scale upon the retardation plate with the zero or base reference mark of the scale accurately located in the transverse plate of zero birefringence of such a retardation plate, and with the positive and negative values on the scale positioned to properly indicate positive and negative values of birefringence provided by the corresponding portions of said retardation plate. Applicants have found that extremely accurate calibrations may be provided for their retardation plate 44 and accurately positioned thereon in the following manner. Since the wedge shaped member 16 may have its opposite ends accurately measured for birefringent prop-' erties, and such depends upon the thicknesses thereof, they may determine the correct size of scale which may be easily calculated and used therewith. This scale is added to the plane parallel plate 18. Since quartz and like birefringent retardation materials employed for such purposes are not easily engraved or etched to give clear markings and numbers for such small sizes as are desirable for the scale to be used on applicants retardation plate, other means or methods of marking are desirable and excellent results may be obtained by coating that portion of the upper surface of member 18 upon which the scale is to ultimately appear with a semitransparent layer of metal, such as platinum.

rhodium or iridium, by a well-known sputtering process performed in a high vacuum. This semitransparent layer of metal is thereafter engraved in such a manner as to remove only a portion of the thickness of the material so disposed on the member 18 and thereby form the individual calibrations, numbers and the like forming the graduated scale.

A scale so formed is indicated in Fig. 5 by numeral 88 and this scale is provided with a zero or base reference mark 80, and positive and negative values upon the scale at opposite sides thereof. The plates I8 so formed with the scale 88 of such size as to correspond with the amount of taper of the wedge member 18 is thereafter placed upon the wedge shaped plate 16 with a layer 82 of adhesive, such as Canada balsam, between said members. The members are then longitudinally shifted relative to one another, while under observation in suitable optical equipment provided for the purpose, until the zero or base reference mark 88 has reached the transverse plane through the wedge shaped member 16 having an equal thickness and is there held until the adhesive has hardened or set. At the plane of the mark 98 the total birefringence of the two superimposed members 18 and 18 will equal zero. In this manner the zero or base reference mark may be easily and exactly located at the plane of zero birefringence.

When such a retardation plate is employed in a polarizin microscope, light being reflected by a mirror 92 of a microscope through a polarizer 93 and suitable condensing lens system 84 and then passing through the slide l8 and the microscope objective 96 will strike the retardation plate 44 so that this plate and the scale 88, as well as the reticule 64 may be viewed through the eyepiece 48. Since the analyzer 66 is positioned between the reticule and the eyepiece birefringence in the specimen on the slide I8 may be observed and the the amount of birefringence may be measured on the scale 88 when the slide 32 is properly adjusted. The slide 32 will be in proper adjustment to indicate the amount of positive and negative birefringence of the specimen when it has been moved to a, position showing a dark line or field traversing the slide and is in alignment with the cross hair 84a of the reticule 64.

From the foregoing it will be seen that we have provided simple, efficient and economical means and a process for obtaining all the objects and advantages of the, invention.

Having described our invention, we claim:

1. A retardation plate of the character described comprising a first plate-like member formed of quartz, said member being relatively long and in the shape of a uniformly tapering thin wedge to provide substantially uniformly varying birefringent retardation properties throughout an elongated portion thereof, a second plate-like member formed of quartz, said second member being relatively long and thin and having plane parallel opposed surfaces to provide substantially uniform birefringent retardation properties throughout an' elongated portion thereof, the parallel surfaces of said second member being spaced so as 'to provide a thickness therein equal to the thickness of -a transverse plane intermediate the ends of the elongated portion of said wedge shaped memher, said wedge shaped member and plane parallel member being so disposed in adjacent faceto-face relation relative to each other that their optical axes are at right angles, a graduated scale having a base reference mark associated therewith for indicating zero birefringence carried by said elongated portion of said plane parallel member with said base reference mark located intermediate the ends thereof, and means securing said members together with said base reference mark located directly opposite the said transverse plane of said wedge shaped member.

2. A retardation plate of the character described comprising a first elongated plate-like quartz member, a second elongated plate-like quartz member arranged in superimposed relation relative thereto, said first member being in the form of a relatively thin wedge to provide substantially uniformly varying birefringent retardation properties throughout the length thereof, said second member being relatively thin and having plane parallel opposed surfaces to provide substantially uniform birefringent retardation properties throughout the length thereof, the parallel surfaces of said second member being spaced so as to provide a thickness therein equal to the thickness of said first member at a transverse plane located intermediate the ends thereof so that the birefringent retardation properties of said second member are equal to the birefringent retardation properties of said first member at said transverse plane, said members being so arranged in adjacent relation that the optical axis of one member extends longitudinally and the optical axis of the other member extends transversely of said members, a coating material upon one of said parallel surfaces of said second member, a scale formed in said coating material intermediate the endsthereof and having a base reference mark associated therewith for indicating zero birefringence, and an adhesive for fixedly securing said members together with said base reference mark exactly located in said intermediate transverseplane of said first member.

3. The method of forming a birefringent retardation plate comprising forming on a first member of birefringent material an elongated relatively thin plate-like portion having opposed tapered surfaces for providing substantially uniformly varying birefringent properties therein, forming on a second member of birefringent material an elongated relatively thin plate-like portion having opposed plane parallel surfaces for providing substantially uniform birefringent properties therein, the plate-like portion of said second member being of such a thickness that the birefringent properties provided thereby will be equalled to the birefringent properties provided at a transverse plane intermediate the ends of the elongated plate-like portion of said first member, measuring the birefringence at two points on said first member located a predetermined longitudinal distance apart for determining the rate of change of birefringence therealong, forming a scale having a base reference mark for indicating zero birefringence on said second member intermediate the ends of the plate-like portion thereof with the spacing of the graduations of the scale determined by said rate of change in. said first member, applying a cement to one of said members, placing said members together with said cement disposed therebetween and with the optical axes of said members positioned at right angles to each other, moving one of said members relative to the other so as to position said base reference mark in exact alignment with said transverse plane of said first member, and maintaining said members in such adjusted position until a set has occurred in the cement joining said members.

4. The method of forming a birefringent retardation plate comprising forming on a first member of birefringent material an elongated relatively thin plate-like portion having opposed tapered surfaces for providing substantially uniformly varying birefringent properties therein,

forming on a second member of birefringent material an elongated relatively thin plate-like portion having opposed plane parallel surfaces for providing substantially uniform birefringent properties therein, the plate-like portion of said second member being formed of such a thickness that the birefringent properties provided there- 'by will be equal to the birefringent properties provided at a transverse plane intermediate the ends of the plate-like portion of said first member, measuring the birefringence at two points on said first member located a predetermined longitudinal distance apart for determining the rate of change of birefringence therealong, forming a layer of coating material on the plate-like portion of said second member, providing a scale having a base reference mark for indicating zero birefringence in said layer intermediate the ends thereof and with the spacings of the graduations of said scale determined by said rate of change in said first member, applying a cementing material to one of said members, placing said members together with the cementing material disposed therebetween and'with the optical axes of said members positioned at right angles to each other, moving one of the members relative to the other so as to position said base reference mark in exact alignment with said transverse plane of said first member, and maintaining said members in such adjusted position until a set has occurred in the cementing material joining said members.

DAVID D. LOWBER. JOSEPH D. REARDON. PHILIP W. COLLYER.

REFERENCES CITED The following references are of record in the file of this patent: 

